As a typical method of manufacturing an optical recording medium, i.e., an optical disk, photopolymerization has been used hitherto. Photopolymerization is a method in which a flat plate of plastic, glass, or the like (referred to as a "flat substrate") is prepared, a photo curing resin, which is hardened by light energy such as ultraviolet rays, is filled between the flat substrate and a stamper, and the photocuring resin is hardened by irradiation with ultraviolet rays.
A conventional photopolymerization method is disclosed in, for example, Japanese Unexamined Patent Publication No. 53-86756. This publication describes a method in which a master (the "master" corresponds to a stamper hereinafter in this description) is formed of nickel by electroforming, and a pattern is transferred from the master onto a flat substrate made of polymethyl methacrylate, polycarbonate, or the like, by using an ultraviolet-curing resin.
Japanese Unexamined Patent Publication No. 5-62254 discloses a method in which silicon is used as a master, and a pattern is transferred from the master of a silicon wafer, which has an uneven pattern formed by etching, onto a flat substrate by using an ultraviolet-curing resin.
In the above conventional photopolymerization methods, a resin plate formed of polycarbonate by injection molding or extrusion is used as the flat substrate in order to improve mass productivity.
Since polycarbonate has a considerably strong optical anisotropy, however, the optical properties of a disk, which is obtained by working a sheet material formed by extrusion, vary depending on the reading direction of a pickup. In order to obtain stable reproduced signals, it is necessary that the optical properties not vary even when the reading direction of the pickup changes somewhat. Consequently, it is impossible to use polycarbonate as a material for the optical disk.
In contrast, when acrylic having a weak optical anisotropy is used instead of polycarbonate, the optical properties of a disk obtained by extrusion and working described above are improved. Acrylic is, however, highly hygroscopic, and thereby tends to cause deformation, such as warping of a disk. Moreover, when a reflecting film, a recording film, or the like is formed on the disk surface by vacuum forming, it takes substantial time to remove gas (in particular, moisture) from the disk.
In order to solve the above problems, it is preferable to use a polyolefin copolymer as a material for a resin plate that satisfies the requirements of desirable optical anisotropy, constant low hygroscopicity, and resistance to deformation. Even when polyolefin is used, however, as long as a resin plate is formed by injection molding, the following problems may arise. Therefore, it is necessary to suggest a new method of producing a resin plate. First, when polyolefin is heated to a high temperature for injection molding and is injected into a mold at a high pressure, decomposition or oxidation sometimes occurs, thereby deteriorating the properties of the optical disk. Furthermore, in injection molding, the resin plate sometimes deforms due to changes in pressure in the process of reducing the pressure immediately after molding. Moreover, in injection molding, after resin is heated to a high temperature to be melted, waiting is necessary for the resin to become sufficiently cooled. This may prolong production time, and increase production cost.